The AS15531 databus, also known as MIL-STD-1553 or simply 1553, is an approximately 30 year old technology that defines the electrical and signaling characteristics for 1 Mbps communications over an asynchronous serial, command/response digital data bus on which messages are time division multiplexed among users. The transmission medium is a twisted wire cable pair. 1553 specifies all of the electrical characteristics of the receivers, transmitters, and cable used to implement the bus, as well as the complete message transmission protocol. 1553 is designed for high integrity message exchanges between unattended equipment.
The United States Department of Defense (“DoD”) requires the use of 1553 as the standard for all inter and intra-subsystem communications on all military airplanes, helicopters, ships and land vehicles. Originally used only in mission avionics, 1553 is now used in flight critical avionics, flight control, weapons, electrical power control, and propulsion control. 1553 was originally published in 1973 for use on the F-16 military aircraft program. The current version of 1553 is MIL-STD-1553B (“1553b”), Notice 2, implemented in 1986.
MIL-STD-1553B utilizes an inefficient Manchester II bi-phase signaling scheme over shielded twisted pair cabling with no capacity for communicating faster than its base 1 Mbps rate.
On the other hand, Line Replaceable Units (LRUs) including state-of-the-art equipment devices and munitions, which are retrofitted to an airframe, often require high bandwidth data links between the new equipment to points in the airframe where control or monitoring is performed.
Furthermore, addition of new digital devices to an after market military or commercial aircraft, typically requires new bus wiring or an expanded load on the already heavily loaded aircraft wiring cockpit applications. The retrofitting of an aircraft to add new equipment by rewiring is not only a very complex process, which requires many months of modification time and involves substantial expenses, but also the new wiring adds weight to the aircraft and takes up space, which is always disadvantageous in any airframe design, especially for high performance airframes in which maneuverability is important.
The use of various technologies to increase the data transfer capacity of existing AS15531 networks without rewiring has been investigated by the Society of Automotive Engineers (SAE) Avionics Systems Subcommittee (AS-1A), in cooperation with SBS Technologies Inc. Results of their investigation are described in a white paper entitled “The Use of Discrete Multi-Tone (DMT) Signaling for Data Transmissions on Existing AS15531 Networks”, published on 15 Aug. 1998, which is incorporated herein by reference. The paper indicates that, Digital Subscriber Line (DSL) coding techniques, such as Carrier-less Amplitude/Phase (CAP) coding and Orthogonal Frequency Division Multiplexing (OFDM), closely related to Discrete Multi-Tone (DMT) coding present themselves as particularly suitable examples of overlay technologies. This paper, however, fails to indicate the feasibility of an operable system that would allow existent 1553 networks to operate in their multi-drop, dual-redundant architecture, at a data transfer rate above 1 Mbps and signaling frequencies above approximately 10 MHz.
According to the preferred embodiment disclosed in the co-assigned U.S. patent application Ser. No. 11/419,742, OFDM modulation is used to better utilize the available bandwidth on the bus, creating an “overlay” network to operate concurrently and without disturbing existing 1553 communications. As illustrated in FIG. 1, this is accomplished by configuring OFDM signals with little energy, or low Power Spectral Density (PSD), in a 1553 high-energy frequency band (0 to F1553) and with a relatively constant Power Spectral Density (PSD) in a 1553 low-energy frequency band (F1553 to FOFDM). In other words, the OFDM waveforms are configured to utilize the frequency band from F1553 to FOFDM, where 1553 side lobes of a given 1553 system are relatively low. In addition, the OFDM signals have little energy in the 0 to F1553 band, reducing the interference of OFDM communications on existing 1553 communications. However, within a system comprising signaling as illustrated in FIG. 1, 1553 sidelobes in the OFDM frequency band have sufficient energy to interfere with the OFDM signals thus limiting the data throughput of the OFDM overlay networks and therefore of the entire communication system.
To summarize, within a non-1553 data communications overlay network, non-1553 signals are susceptible to interference from the 1553 signals being communicated over the same physical medium.
1553 signals are defined as signals in accordance with 1553 standard signaling schemes, including but not limited to primitive Manchester II bi-phase signaling. Non-1553 signals are defined as any signals that can be differentiated from 1553 signals either in frequency domain, time domain, Laplace domain, or by any other method obvious in the art.
There is a need in the art for efficient methods of attenuating the interference of 1553 signals on non-1553 signals within a non-1553 network overlayed without rewiring on a 1553 communication system, for increasing throughput of the overall communication system.